The molecular structure of Cr[(CH2)2PMe2]3: dimethylphosphonium-bis-methylide chromium compounds as inner-phosphonium-alkyl-ate- or 2-phospha-allyl-complexes?

The molecular structure of the chromium(III) complex Cr[(CH2)2PMe2]3 (1a) was determined by X-ray crystallography. The bonding mode of the chelating dimethylphosphonium-bis-methylide ligand is discussed either as a part of an inner-phosphonium alkyl-ate-complex or as a 2-phospha-allyl system. In principle it seems possible to extend this consideration also to the bridging dimethylphosphonium-bis-methylide ligands in the chromium(II) complex Cr2[(CH2)2PMe2]4. Regarding the electron delocalization in such ligands the structural parameters of 1a are compared to the results of recently described quantum chemical calculations.